1. Field of the Invention
The present invention relates to an electroluminescent element (hereinafter abbreviated as EL element), and more particularly relates to an electroluminescent element having improved voltage withstanding properties, which is suitable for position indication of a switch member or the like by bridging thereacross.
2. Description of the Prior Art
Hitherto, EL elements of various structures have been known. For instance, there are ones formed by successively applying an electrode layer, a dielectric layer, a luminescent layer and an electrode layer on an insulating substrate, or ones formed by, successively, a dielectric layer, a luminescent layer and an electrode layer on an electrically conductive substrate serving as another electrode layer. In these EL elements, porcelain enamel, a mixture of powdered titanate (e.g., barium titanate) and glass, and the like are usually used as the dielectric layer.
It has been known that the EL element of this type can be used as a position indicating lamp without particular difficulty as long as it is connected across voltage terminals of a power source as a load. However, one intends to use the EL element for the indication of switch members in lighting equipment by connecting the element in parallel between the switching contacts of a switch member and energizing it only under the switching-off condition of the switch member, a transient phenomenon is caused due to the presence of load inductance at the time of switching-off, so that higher voltage (2-3 KV) is applied to the element and it is apt to cause breakdown.
Therefore, it is desired to develop EL elements having higher voltage withstanding properties. For this purpose, it is known that a semiconductor layer is inserted between the dielectric layer and the electrode layer in order to improve the voltage withstanding properties without sacrifice of luminous intensity at a constant applied voltage. However, the thickness of the semiconductor layer is considerably large as compared with the thickness of the dielectric layer, so that there is a tendency to unduly increase the thickness of the EL element.